Existing data on hidden variation, much of it gathered in my lab, is anomalous in two respects: 1. Too few heterozygotes are observed; 2. Most alleles are ery rare. And yet the variation is heritable. One hypothesis consistent with these results is that much of the variation reflects post-translational modification of proteins by other polymorphic loci. Recent data on XdH strongly supports this possibility. An extensive survey of "hidden" electrophoretic variation in Drosophila melanogaster is proposed, using several high-resolution analytic techniques which have as a common basis gel sieving analysis. For several selected loci, an attempt will be made by examining co-isogenic lines to locate all variants to the appropriate chromosome, and to map them. This will provide a direct determination of the proportion of the heritable variation resulting from second-site post-translational modification. Kinetic analysis will be carried out to assess the degree to which variation in shape entails functional difference.